In a traditional terrestrial environment, gravity and atmospheric pressure are often exploited to drive liquids from one vessel to another. For example, a source container may be placed higher than a destination container. The source container may be open to the ambient atmosphere such that gas pressure in gasses above the liquid level in the container may be equal to atmospheric pressure. A proximal opening to a conduit may be placed below the level of liquid in the container. Thus, when liquid is withdrawn from an opposite, distal opening of the conduit, atmospheric pressure may be relied upon to push liquid into the proximal end to replace the quantity of liquid that is withdrawn at the distal end.
Similarly, in the terrestrial environment, the force of gravity generally causes the liquid in a container to accumulate in a lower portion of the container or of another component of a fluid-handling system. Gasses such as air or vapor, being generally less dense than the liquid, accumulate in an upper portion of the container or component, above a liquid level. Therefore, liquid may be passively drained from the container or component via an opening that is placed below the liquid level. Structures to vent air bubbles, or that are otherwise open to the ambient atmosphere, may be placed above the liquid level.
In some cases, it may be desirable or advantageous to operate a process involving liquids in a microgravity environment. For example, an experimental or industrial process may be operated in a spacecraft in earth orbit or on a spacecraft travelling outside of earth orbit. The process may be controlled by a human operator on the spacecraft or remotely located, or may be controlled automatically.